Although various types of seals or gaskets have been widely used in the past to bond the edges of the plates of a liquid crystal display together so as to contain the liquid crystalline material therebetween and provide a hermetic seal about the liquid crystalline material, none have heretofore provided a good barrier against vapor penetration into the cavity in conjunction with having good mechanical durability in combination with being made from low cost materials that are easily handled and applied.
Such seals are typically deposited as a fluid coating about the peripheral edges of at least one but more commonly about both of the plates with the plates then pressed together to provide a cavity within the coated edges between the plates for containing the liquid crystalline material. The thickness of the cavity between the plates is typically in the order of several microns to about 100 microns in thickness and the uniformity of the thickness of the cavity between the facing surfaces of the plates is typically controlled by the use of spacers having a controlled thickness between the plates or in some instances by incorporating spacing material such as glass beads randomly in the sealant material or in a controlled manner such as disclosed in U.S. Pat. No. 3,919,452. Commonly at least one opening is provided through the seal for introducing the liquid crystalline material into the cavity either before the plates are pressed together or afterwards for example by drilling or pressing an object of suitable thickness and length through the seal to provide the opening. More commonly, at least two openings are provided through the seal in the manner described above and positioned so as to enable the liquid crystalline material to be introduced into the cavity without air entrapment or creating of a vacuum. After the cavity is filled with the liquid crystalline material, the openings are sealed with a material that preferably will bond to the sealant material and, like the sealant material, will not contaminate and will be chemically inert with respect to the liquid crystalline material. In some instances such openings may not be required where, prior to curing, the plates are first pressed together with the sealant material in place therebetween and then are able to be sufficiently separated along a portion of at least one edge to provide an opening through which the liquid crystalline material can be poured and afterwhich the plates are able to be again pressed together and the assembly cured under heat. Since the facing inner surface of the plates of a liquid crystal display are coated with a transparent electrically conductive material such as tin oxide or indium oxide that is typically etched into a plurality of discrete image segments with individual conductive leads reaching to the edge of one of the plates, accommodations are normally made such that the seal does not interfere with electrical power connections to the leads. In addition, it is common practice to deposit an organic surfactant material such as polyvinyl alcohol over the conductive coating that can be rubbed to provide sufficient homogeneous (parallel) alignment of a nematic liquid crystal having positive dielectric anistrophy. Likewise, means, such as a highly conductive plug of low electrical resistance such as silver, is often provided through the seal to transmit electrical power to the conductive coating in the inner surface of the plate not having the power leads so as to transfer electrical power to the plate when electrical power is applied to the plate having the power leads. Although not hereinafter referred to or shown in the drawings, it is to be understood that the seal of the display of the present invention may be adapted to provide access to such power leads and such transfer of power to the plate opposite the plate having the power leads in any suitable manner. Depending upon the materials from which the liquid crystal display seal is made, the seal may or may not be cured under heat for a prescribed period of time after the liquid crystalline material has been introduced into the cavity by pouring or through the openings previously described.
An example of a liquid crystal display seal made from tetrafluoroethylene-ethylene-copolymer or a chlorotrifluroethylene-ethylene copolymer is disclosed in U.S. Pat. No. 3,871,746. U.S. Pat. No. 3,990,781 discloses a liquid crystal display seal made from a dispersion of flurocarbon and an organosilane coupling agent. Although such seals may be used to advantage, their use is generally limited for they require the use of costly flurocarbonated materials and generally involve the time consuming task of having to provide the previously described openings through which the liquid crystalline material is to be introduced as well as requiring a curing temperature in excess of 240.degree. C. that would preclude using the rubbed polymeric coatings previously described and require coatings such as are provided by the expensive process of sputtering a silicone-oxide coating or the like onto the conductive coating.
U.S. Pat. No. 3,926,502 discloses a liquid crystal display seal made from a low melting point glass. Although such seals may provide a satisfactory hermetic seal, they require expensive application equipment and difficult handling procedures as well as requiring the previously described openings through the seal through which the liquid crystalline material is introduced and are unable to permit the pouring technique previously described. Further, such glass seals are generally known to be brittle and possess poor resistance to vibration as well as precluding the use of organic surfactant coatings on the facing surfaces of the display plates due to the high temperature required in the process of applying such glass seals.
U.S. Pat No. 3,994,568 discloses a liquid crystal light shutter display gasket made from a thermoplastic phenoxy resin that may contain high molecular weight epoxy resins that may have their widely separated epoxy rings reacted to form a "slightly" thermosetting adhesive sealer which will behave in a similar manner to the all thermoplastic phenoxy. It has been found however that such phenoxy when either uncrosslinked or crosslinked only to the extent that it still behaves as a thermoplastic material is unable to provide a good barrier against the penetration of water vapor into the cavity containing the liquid crystalline material. Water vapor penetration into the cavity is undesirable in that it has been found to produce spurious electrical paths within the cavity and cause a diffusion of the image produced and thereby cause what is known as "blooming" in the trade.
Examples of cyanoacrylate and polybutadiene materials that have been used as a sealant for the previously described openings through the seal of a liquid crystal display are disclosed in U.S. Pat. No. 4,007,077. Although such materials may be suitable for the purpose described, they are generally unable to provide the ability to hermetically seal in conjunction with having high resistance to vapor penetration and good mechanical durability.